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EQUILIBRIUM OF A PARTICLE                    15
the equilibrium of the shaded part of the rope in Fig. 14a. The shaded part is acted upon              -_^__^____^
by the adjoining sections of the   ***~(       E.M..........................0"'—*
rope. Therefore we consider the shaded part alone and represent the actions of the adjoining parts by the forces - F and F7 as shown in Fig. 14b.
20.   Definition of Force. — Force is a vector magnitude which represents the action of one body upon another.    The interaction between two bodies takes place across an area, while the forces which represent them are supposed to be applied at one point.    Therefore the introduction of the idea of force presupposes the simplification of dynamical problems which is obtained by considering bodies as single particles, or as a system of particles.
21.   Internal Force. —A force which represents the action of one part of a body upon another part of the same body is called an internal force.
22.   External Force. -A force which represents the action of one body upon another body is called an external force.
23.   Unit   Force. — The  engineering unit  of force among English speaking people is the pound.    The pound is the weight, in London, of a certain piece of platinum kept by the British government.
24.   The Law of Action and Reaction.—The fundamental law of Mechanics is known as the law of action and reaction. Newton (1692-1727), who was the first to formulate it, put the law in the following form.
"To every action there is an equal and opposite reaction, or the mutual actions of two bodies are equal and oppositely directed."
Let us apply this law to the interaction between a book and the hand in which you hold it. Your hand presses upward upon the book in order to keep it from falling,